Yes, I think I don't understand what you means by "runner".
For me, the runner is the program called "Solarus.exe" on Windows, and "solarus-run" on Unix (and probably Mac?). And even if this program is kind of light (the engine is not the heaviest game engine I saw

), I don't see the interest to include both of them in one archive.
The quest is already accessible for all of the OS-es. So you still can work on an OS, boot up on another and work on it too. The only thing OS-specific is the program, and as I said, I don't see the point to distribute all the programs versions for each OS-es, again because a lot of people don't care to get OS they don't have.